1. FIELD OF THE INVENTION
This invention is in the field of cementation processes for precipitating metals from solution, particularly copper, by carrying out the cementation reaction in an oscillatory reactor at least partially filled with agitation bodies of controlled size to promote the progress of the reaction and to conduct it more efficiently.
2. DESCRIPTION OF THE PRIOR ART
Copper ores of the oxidized type can be reduced by hydrometallurgical processes in which the ore is leached without being concentrated, usually with dilute sulfuric acid. A solvent is employed which does not dissolve any silver or gold present. After leaching with the solvent, the resulting copper ions in solution may be precipitated by the addition of reactive metals into a form known as cement copper.
There is a widely developed method in hydrometallurgy for the cementation of metal from a leach solution utilizing a zinc precipitation. In this method, zinc dust or zinc powder is added as the cementation medium. The use of this type of process, however, in conjunction with solutions which may also contain cobalt, nickel, or other ions presents a difficulty because the reaction speed is very low. This results because of the production of an immunizing coating such as zinc sulfate on the substantial surface of the fine-grained cementation medium. In order to overcome this reduction of reaction rate, a substantial excess of zinc powder or zinc dust above the stoichiometric quantity can be introduced. In addition, a relatively low pH in the range of 4.2 to 4.5 may be maintained through continuous addition of acid. Finally, the charge may be agitated with a high energy agitation means.
There is a further disadvantage in this method in that a part of the zinc dust particles after formation of a sulfate coating, mixes with the cemented metal and is separated out with the metal in admixture. This contamination of the cemented metal requires additional expensive processing steps in order to recover the metal in substantially pure form.
Carrying out cementation processes in an oscillating container at least partially filled with pieces of a cementation medium is known, for example, from German Laid Open Specification No. 24 57 660 and U.S. Pat. No. 4,152,143. The precipitation medium consists of iron bodies such as iron scrap and/or iron granulate. The cementation medium is present in the form of bulky, solid bodies which act to transfer the shaking movements produced within the oscillating reactor because of their mass to the entire space within the reactor. There thus results a permanent abrasion process on the surfaces of the cementation medium which are thereby "activated" in that they are constantly being liberated from passivating metal coatings and because of this, the cementation reaction proceeds continuously with high electrochemical potential and therefore with the highest possible reaction velocity.
These circumstances do not occur in the case of cementation procedures from solutions using the addition of a fine-grained metallic cementation medium. It was found that the utilization of the oscillating reactor brings about no improvement in the reaction velocity, and that other difficulties such as the necessity of using large excesses of cementation media as well as the addition of acid are still necessary.
Merrill U.S. Pat. No. 684,578 describes the addition of pulverized silica, emery, corundum or the like to zinc to act as a precipitant for recovering precious metals from cyanide solutions. The function of the pulverized material is to keep the surfaces of the zinc particles mechanically clean, and thus avoid passivating layers on the zinc particles. These pulverized materials have the disadvantage that they mix with the cement material and are difficult to remove by conventional means after the refining process.